I don't think there's a problem if a partner wants to go outside the relationship for sex, but doesn't want their partner to do so.
As far as I`m concerned if one partner wants to have some NSA fun with others but denys and trys to prevent their partner from doing the same, well that person is being incredibly selfish and I for one would not stand for that kind of shit under any circumstance.
Now if the other partner is okay with the arrangement then that is different, but lets say for arguments sake that one day this same partner who has been sitting on the sidelines while the missus has been going out and about getting her fuck on decides that he wants to indulge in a bit of fun as well, but then has his mate tell him that she wouldn`t be comfortable with that, well I`m sorry but that is bullshit and can in no way be justified imo.
The reason I told the guy to test his girl by asking for something in return if he allows her to indulge in this fantasy is so that he can see for himself what her feelings are for him, if she is against the idea of quid pro quo then she isn`t to be trusted and he should move on, however if she says okay because its only fair you get yours if I get mine then that relationship has potential as she is being considerate and shows that she cares about his fantasys and will try and make them a reality together.
I hope I`ve explained my position clearly here.
The problem comes in when a partner EXPECTS this to happen.
I don`t quite understand what you mean here, can you elaborate?